Torsional motion and elasticity of the deoxyribonucleic acid double helix and its nucleosomal complexes.

Torsional thermal oscillations of the DNA double helix within the electron paramagnetic resonance (EPR) time scale (10(-10)-10(-3) s) as indicated by a rigid, intercalating probe are much smaller in the spacer segment between nucleosomes in chromatin than in long, free DNA molecules. Still smaller DNA oscillation is indicated in intact nuclei and yet smaller if the nuclei have been treated with glutaraldehyde. The values of EPR measurements are not affected by the loading density of probe. If the probe were capable of substantial oscillations or movement different from that of the helix, those oscillations would be expected to dominate the spectra when movement of the helix is restrained. We conclude that the correlation time for torsional movement of free DNA inferred from EPR spectra is characteristic of the double helix and that there is no significant independent motion of the probe. The correlation time for the DNA double helix in molecules longer than approximately 500 base pairs is close to 30 ns, corresponding to an elastic constant of 1.5 X 10(-19) ergs cm for deformation by twisting. The motions observed in chromatin are consistent with a model in which spheres of 50-60-A radius are connected by simple elastic rods with the length of spacer DNA and the same elastic constant. The spin-labeled ethidium probe has been characterized in detail by nuclear magnetic resonance, infrared, fluorescence, and visible light spectroscopy. The binding equilibria are consistent with the hypothesis that strongly immobilized probe molecules are preferentially bound to spacer DNA.